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The valves of some bivalve shells are formed exactly alike, and others are very different; the one being smooth, the other rugose; one flat and another convex; and often one is shorter than the other.

The shells of the Mya, Solen, Tellina, Venus, and others, have in general both valves alike, while those of the Spondylus, Ostrea, and Anomia, have in general dissimilar valves. The first of these kinds are called equivalve, and the latter inequivalve.

Equilateral shells, are those whose sides are alike, as in the shells of the genus Pecten. Plate II. fig. 2, and Plate VII. fig. 14. This is also exemplified in the Pectunclus.

Inequilateral valves are shells whose sides are unequal; and of different shapes, as in the Mactra, Donax, &c.

Summit is the most elevated point of that part of the shell in which the hinge is placed. Plate I. fig. 4, i k.

In naming this the summit we do not follow the axiom of Linnæus, but because we consider it more properly the summit of the shell than the opposite extremity.